Manufacture of micro-crystalline wax impregnated electric cables



Nov. 24, 1959 A. KING 2,914,429

MANUFACTURE OF MICRO-CRYSTALLINE WAX IMPREGNATED ELECTRIC CABLES Flled March 3, 1952 2 Paper insulation impregnated with a normally plastic sol/d consist/n got 3 Meta/l/zed paper screen m/cro-crysta/hhe petroleum wax and cable impregnat/ng mineral 017.

L ead shea th 4 6 Steel wire armour/n9 Jute serving 7 5) Mo maa ATTORNEY United States Patent MANUFACTURE OF MICRO-CRYSTALLINE WAX IlVIPREGNATED ELECTRIC CABLES Albert King, Bexley Heath, England, assignor to British Insulated Callenders Cables Limited, London, England, a British company Application March 3, 1952, Serial No. 274,596

Claims priority, application Great Britain March 21, 1951 Claims. (Cl. 117231) This invention relates to insulated electric cables having a dielectric of impregnated fibrous insulating material, generally paper, and in particular to fully impregnated non-draining cables of this kind.

By stating that the cable is fully impregnated it is intended to imply that the cable is as full of impregnant as is possible, the paper or other fibrous material being impregnated and the spaces between the turns and layers thereof being filled with compound, the object being to ensure that the possibility of the existence of voids within the sheath which surrounds the dielectric is reduced to a minimum, and by a non-draining cable is meant a cable such that when a vertical. length of 30 feet or more ofthe cable, with open ends, is heated to its maximum rated conductor working temperature for 7 days or more the amount of impregnating compound drained from the cable does not exceed 0.1% by volume of the cubical capacity of the cable sheath.

In the specification of my U.S. Patent No. 2,586,345

there is described and claimed a fully impregnated cable in which migration of impregnant when the cable is run vertically or laid on steep gradients is avoided by employing an impregnant which is a normally plastic solid consisting of micro-crystalline petroleum wax having a melting point of 80 C. or higher or a compound which is a normally plastic solid consisting of micro-crystalline petroleum wax having a melting point of not less than .80 C. and of cable impregnating mineral oil or one of a group of two plasticisers consisting of polyisobutene (polyisobutylene) having an average molecular weight of 20,000 to 100,000 and rosin or a mixture of such oil with one or both of such plasticisers, the proportion of micro-crystalline wax in the compound being not less than 55% by weight.

By the term micro-crystalline petroleum wax used above and hereinafter is meant wax of the petrolatum group or petroleum ceresin group of petroleum waxes (as defined and classified in the Journal of the Institute sulating oils in the Journal of the Institution of Electrical Engineers, part II, pages 3-64, vol. 90, 1943. They vary in viscosity from oils having a viscosity of about 11,000 centistokes or higher at 0 C. to the so-called hollow core cable oil having a viscosity of about 140 centistokes at 0 C.

We have now foundthat a particular grade of microcrystalline petroleum wax not hitherto used as a cable impregnant, namely, micro-crystalline petroleum wax having a melting point of about 88 C. and needle pene- 2,914,429 Patented Nov. 24, 1959 "ice tration values (as determined in accordance with Method IP-49/ 46 of the Institute of Petroleum) of about 5 at 25 C. and about 35 at 70 C. is especially advantageous as an impregnant, or a constituent of the impregnant, for a non-draining cable and can be used in amounts varying from about 48% to 100% depending upon the operating temperatures of the cable and the factor of safety required. Although it will generally be technically more advantageous to use only micro-crystalline petroleum wax of the specified grade as the impregnant, or the wax constituent of the impregnant, we may, where economic or other reasons make it desirable, blend such wax with micro-crystalline petroleum waxes having higher penetration values at high temperatures to obtain a blended wax having a melting point of 85-90 C. and needle penetration values varying from about 5 to 6 at 25 C. to between 3550 at 70 C. The minimum amount of this blended wax that can safely be used will be higher than the figure of 48% specified for wax having a needle penetration value of 35 to 70 C., and will be not less than about 50%. The present invention therefore comprises fully impregnated non-draining cable in which the impregnant within the paper (or other fibrous insulating material) and in the interstices between the turns and layers thereof is a normally plastic solid consisting of micro-crystalline wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 or a blend of micro-crystalline wax having a melting point range of 8590 C. and needle penetration values varying from about 5 and 6 at 25 C. to between 35 and 50 at 70 C. or is a compound which is normally a plastic solid and consists of such micro-crystalline petroleum wax and of cable impregnating mineral oil or one or both of the group of plasticisers consisting of polyisobutene having an averagemolecular weight of 20,000 to 100,000 and rosin or a mixture of such oil with one or both of such plasticisers, the proportion of micro-crystalline wax in the compound being not less than about 48% by weight when only wax of the grade having a needle penetration value of 35 to 70 C. is used and not less than about 50% by weight when the specified blend of wax is used.

We have found that wax of the grade specified is better able at cable operating temperatures to retain in its crystal structure its natural oil content and, when present, the added oil or plasticiser than are waxes having much higher penetration values at high temperatures,

such as those described in Examples 1 to 7 of the complete specification of the aforesaid patent of which the needle penetration values at 25 C. are about 6 and at 70 C. vary from about to 150. Our experiments have shown that cables impregnated with wax of the grade specified herein or with compositions containing it in large proportions, have less tendency to drain when run vertically or laid on a steep incline and operating at a temperature lower than the melting point of the wax or compound than have cables impregnated with microcrystalline waxes having higher penetration values at high temperatures or with comparable compounds containing them. Cables according to the present invention can therefore be manufactured to operate at higher temperatures without draining or with a greater margin of safety at the same temperatures than is the case with cables in which the impregnant or a major constituent thereof is micro-crystalline wax having relatively high penetration values at high temperatures. Alternatively, since microcrystalline wax is a relatively expensive material, they can be manufactured at less cost than cables designed to operate at the same temperature with the same factor of safety as regards non-draining character- 'istics and using as the impregnant or major constituent of the impregnant micro-crystalline wax having relatively high penetration values at high temperatures.

Micro-crystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about at 25 C. and about 35 at 70 C. is sold by the British Sun Oil Company Limited under the trade name Sunoco 985 Yellow Micro-crystalline Wax. This wax is hereinafter referred to as Sunoco 985.

It is explained that we have found that cables impregnated with micro-crystalline waxes and impregnating compounds containing them are not necessarily free from draining tendencies merely because the impregnants have melting points higher than the temperatures at which the cables are tested or operated. However in the course of our experiments in connection with non-draining cables we have developed an empirical test (which we have termed the absorption test) which we have found to give a reliable indication of the non-draining characteristics of impregnated paper insulated cables under operating conditions. In this test the absorptive property of the paper (or other fibrous cable dielectric material) is matched against the ability of waxes and impregnatingcompounds containing them to retain their oil and plasticiser contents. The test comprises placing a cast sample of the wax or wax impregnating compound of standard dimensions in contact with a sheet of paper of the grade to be used in the manufacture of the cable dielectric and maintaining the assembly in a horizontal position at a given raised temperature for a given period of time during which oil from the sample is absorbed by the paper to an extent dependent upon the characteristics of the impregnant and upon the chosen conditions of test, and observing the extent of the absorption. The perimeter of the oily patch of paper produced is substantially the circumference of a circle. The difference between this perimeter and the circumference of the sample under test we term the abscription index of the sample.

The absorption index figures quoted hereinafter have been determined under the following test conditions:

(1) Samples of impregnant were prepared by casting in a watch glass having a diameter of 3.8 cuts, a circumference of approximately 12 cms. and a depth of concavity of 0.4 cm.

(2) The fiat surface of the cast sample was placed over the centre of a rigidly held six inch square of cable insulating paper.

(3) The cable insulating paper was of sulphate wood type, and had a thickness of 5.25 mils (0.13 mm), an

apparent density (65% relative humidity and 20 C.) of I 0.78 and a Gurley porosity of 150 seconds per 100 cc. (4) The temperature and duration of the test were respectively, 170 F. (76.7 C.) and 16 hours.

We have found that when the micro-crystalline wax impregnant of a paper insulated cable has a melting point of not less than 75 C. and an absorption index Example 1 Parts by weight Sunoco 985 60 Hollow core cable oil (oil filled cable oil) of viscosity 140 centistokes 0 C. and 7.3 centistokes at 60 C 40 This compound has a melting point of 82 C. and cone penetration values of 87 at 25 C. and 120 at C. and an absorption index of 7. It will be seen therefore to be an example of a compound which can be used to provide cable having a high factor of safety as regards non-draining characteristics at customary cable operating temperatures or which can safely be used in cables operating at higher conductor temperatures than hitherto customary. For instance, it may be used in cables designed to operate with conductor temperatures of C. and somewhat higher.

Example 2 Parts by weight Sunoco 985 48 Hollow core cable oil as specified in Example 1 52 This compound has a melting point of C., cone penetration values of 117 at 25 C. and 175 at 60 C., and an absorption index of 15. It is comparable as regards its non-migratory properties under cable operating conditions with a compound consisting of Parts by weight Micro-crystalline wax having a melting point of 1 C. and needle penetration values of 6 at 25 C. and of 150 at 70 C 60 Hollow core cable oil as specified in Example 1 40 Example 3 Parts by weight Sunoco 985 42 Micro-crystalline wax (melting point C., needle penetration of at 70 C.) 8

Hollow core cable oil as specified in Example 1 50 This compound has a melting point of 82 C., cone penetration values of 95 at 25 C. and 180 at 60 C., and an absorption index of 14. It is rather similar as regards its non-migratory properties to the compound of Example 2.

Example 4 Parts by weight Sunoco 985 34 Micro-crystalline wax (melting point 89 C., needle penetration of 95 at 70 C.) 19

Hollow core cable oil as specified in Example 1 47 This compound has a melting point of 82 C., cone penetration values of 90 at 25 C. and at 60. C., and an absorption index of 12.

This compound has a melting point of 87 C., cone penetration values of 30 at 25 C. and 70 at 60 C. and an absorption index of 3.

This'compound has a meltingpoint'of 85 C., cone penetration values of 46 at 25 C. and:83 at 60 C. and an absorption index of 7.

Example 7 Parts by weight Sunoco 985 98 Refined rosin This compound has a melting point of 88 C., cone penetration values of 4 at 25 C. and 50 at 60 C. and an absorption index of 0.

The cone penetration figures given for the above compounds was determined by Method IP-50/48. Procedure B (cone) of the Institute of Petroleum and the melting points were taken by the Standard Method of Test for Melting Point of Petrolatum, designated D127- 30, described on page 532 of A.S.T.M. Standards, part 2, 1930, published by the American Society for Testing Materials.

The compounds are prepared by mixing hot and are sufiiciently fluid at the ordinary impregnating tempera 'tures, about 130 C., to ensure complete impregnation of the dielectric. At temperatures below the melting points given for the compounds, they are plastic solids which will not flow in the cable at the usual operating temperatures. It will be appreciated from the low temperature penetration figures quoted that the improvements in non-draining properties of cable manufactured in accordance with the present invention are obtained without sacrificing to any, or any substantial, extent the bending characteristics of the cable at usual ambient temperatures. However, in the case of compounds containing considerably higher proportions of Sunoco 985 or similar wax having low needle penetration values at high temperatures, it may be necessary to avoid bending the cable during times when ambient temperatures are abnormally low or to take steps to raise the internal temperature of the cable to a value at which the cable may be bent with safety.

The accompanying drawing which is not to scale shows in cross-section an example of a three-core paper insulated mine shaft cable made in accordance with the invention. Each core comprises a sector-shaped, stranded copper wire conductor 1 on which is built-up a body of insulation consisting of helically lapped paper tapes 2. Surrounding each insulated conductor is a conductive screen 3. This screen, of which the thickness has been exaggerated to show it more clearly, is of metallised paper. Enclosing the three laid up screened cores is a lead sheath 4. In the interstices between the cores and those of the laminated paper body on each conductor and impregnating the paper of which the body is built up is a normally plastic solid comprising micro-crystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and cable impregnating mineral oil, the proportion of such wax amounting to at least 48% by weight of the whole. Over the lead sheath there is a protective layer 5 of impregnated jute on which is laid a steel wire armouring 6 which, in turn, is covered by a second protective layer 7 of impregnated jute.

What I claim as my invention is:

1. In the manufacture of a fully impregnated nondraining paper insulated cable, the method which comprises impregnating the paper of the cable with an impregnating compound which is a normally plastic solid and which comprises as an essential ingredient microcrystalline petroleum wax having a melting point ranging between about C. and C. and needle penetration values ranging from about 5 and 6 at 25 C. to between 35 and 50 at 70 C., the proportion of said microcrystalline petroleum wax in the normally plastic solid being at least about 48% by weight, at least part of said microcrystalline petroleum wax having a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C.

2. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 1, in which all of the microcrystalline petroleum wax has a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and which also contains cable impregnating mineral oil as hereinbefore defined.

3. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 1, in which all of the microcrystalline petroleum wax has a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and which also contains at least one of the group of plasticizers consisting of polyisobutene having an average molecular weight of 20,000 to 100,000 and rosin.

4. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 1, in which all of the microcrystalline petroleum wax has a melting point of about 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. and which also contains cable impregnating mineral oil as hereinbefore defined and at least one of the group of plasticizers consisting of polyisobutene having an average molecular weight of 20,000 to 100,000 and rosin.

5. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 1, in which the microcrystalline petroleum wax is a blend of microcrystalline petroleum wax having a melting point of 88 C. and needle penetration values of about 5 at 25 C. and about 35 at 70 C. with microcrystalline petroleum wax having higher penetration values at high temperatures, the proportion of blended wax in said nor mally plastic solid being not less than about 50% by weight.

6. The method of impregnating the paper of a non draining paper insulated cable as described in claim 5, in which the impregnating compound also contains cable impregnating mineral oil as hereinbefore defined.

7. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 5, in which the impregnating compound also has at least one of a group of plasticizers consisting of polyisobutene having an average molecular weight of 20,000 to 100,000 and rosin.

8. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 5, in which the impregnating compound also has cable impregnating mineral oil as hereinbefore defined, at least one of a group of plasticizers consisting of polyisobutene having an average molecular weight of 20,000 to 100,000 and rosin.

9. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 5, in which the microcrystalline petroleum wax having a melting point of 88 C. constitutes about 42 parts by weight, in which the other microcrystalline petroleum wax having higher penetration values constitutes about 8 parts by weight and has a melting point of about 90 C. and a needle penetration of about 150 at 70 C. and which also has about 50 parts by weight of hollow core cable oil having a viscosity of about centistokes at 0 C. and about 7.3 centistokes at 60 C.

10. The method of impregnating the paper of a nondraining paper insulated cable as described in claim 5, in which the microcrystalline petroleum wax having a melting point of about 88 C. constitutes about 34 parts by weight, in which the microcrystalline petroleum wax having higher penetration values has a melting point'of about 89 C. and a needle penetration of about 95 at 70 C. and constitutes about 19 parts by Weight and which also has about 47 parts by weight of hollow core cable oil having a viscosity of about 140 centistokes at 0 C. and about 7.3 centistokes at 60 C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Publication, Modern Packaging, volume 17, January 1944, number 5, Petroleum'Waxes, pages 48, 49 and 50.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,914,429 November 24, 1959 Albert King It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 2, line 16, for "about 5 to 6 at 25 C." read about and 6 at at line 20, for "value of to "70 0.," read value of 35 at 70 Cm, lines and 41, for "value of 35 to 0." read value of 35 at 70 C, column 3, line 37, for "abscription" read absorption lines 42 and 43, for "depth of concavity of 0,4 cm. read depth of concavity of 0,4 ems, column 5, line 20, for "pounds Was determined" read pounds Were determined Signed and sealed this 22nd day of March 1960.

(SEAL) I Attest:

KARL AXLINE ROBERT c. WATSON Attesting ()flicer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,914,429 November 24, 1959 Albert King It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 16, for "about to 6 at C." read about 5 and 6 at 25 0. line 20, for "value of to 70 0.," read value of 35 at 70 C, lines and 41, for value of 35 to 0." read value of 35 at 70 CD column 3, line 3'7, for "abscription" read absorption lines 42 and 43, for "depth of concavity of 064 cm. read depth of concavity of 0.,4 cms column 5, line 20, for "pounds was determined" read pounds Were determined Signed and sealed this 22nd day of March 1960.

(SEAL) v Attest:

KARL AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents 

1. IN THE MANUFACTURE OF A FULLY IMPREGATED NONDRANING PAPER INSULATED CABLE, THE METHOD WHICH COMPRISES IMPREGNATING THE PAPER OF THE CABLE WITH AN IMPREGNATING COMPOUND WHICH IS A NORMALLY PLASTIC SOLID AND WHICH COMPRISES AS AN ESSENTIAL INGREDIENT MICROCRYSTALLINE PETROLEUM WAX HAVING A MELTING POINT RANGING BETWEEN ABOUT 85*C, AND 90*C, AND NEEDLE PENETRATION VALUES RANGING FROM ABOUT 5 AN D 6 AT 25*C, TO BE- 